In order to produce oil or gas, a well is drilled into a subterranean formation, which may be a reservoir or adjacent to a reservoir. Various types of treatments are commonly performed on a well or subterranean formation. For example, stimulation is a type of treatment performed on a well or subterranean formation to restore or enhance the productivity of oil and gas from the well or subterranean formation. Stimulation treatments fall into two main groups; hydraulic fracturing and matrix treatments. Fracturing treatments are performed above the fracture pressure of the subterranean formation to create or extend a highly-permeable flow path between the formation and the wellbore. Other types of treatments include, for example, controlling excessive water production and sand control.
A well or subterranean formation is normally treated with a treatment fluid. A treatment fluid is typically adapted to be used to resolve a specific purpose, such as stimulation, isolation, or control of reservoir gas or water. “Hydraulic fracturing,” sometimes simply referred to as “fracturing,” is a common stimulation treatment. A treatment fluid adapted for this purpose is sometimes referred to as a “fracturing fluid.” The fracturing fluid is pumped at a sufficiently high flow rate and pressure into the wellbore and into the subterranean formation to create or enhance a fracture in the subterranean formation. Creating a fracture means making a new fracture in the formation. Enhancing a fracture means enlarging a pre-existing fracture in the formation.
To fracture a subterranean formation typically requires hundreds of thousands of gallons of fracturing fluid. Further, it is often desirable to fracture at more than one downhole location of a well. Thus, a high volume of fracturing fluid is usually required to treat a well, which means that a low-cost fracturing fluid is desirable. Because of the ready availability and relative low cost of water compared to other liquids, a fracturing fluid is usually water-based.
When a fracture is formed or extended, fracturing fluid suddenly has a fluid flow path to flow more rapidly away from the wellbore. As soon as the fracture is created or enhanced, the sudden increase in the flow of fluid away from the well reduces the pressure in the well. Thus, the creation or enhancement of a fracture in the formation is indicated by a sudden drop in fluid pressure, which can be observed at the wellhead.
Newly-created or extended fractures tend to close after the pumping of the fracturing fluid is stopped. To prevent the fracture from closing, a material must be placed in the fracture to keep the fracture propped open. A material used for this purpose is referred to as a “proppant.” The proppant is in the form of a solid particulate, which can be suspended in the fracturing fluid, carried downhole, and deposited in the fracture as a “proppant pack.” The proppant pack props the fracture in an open condition while allowing fluid flow through the permeability of the pack.
A number of types of bacteria can be natively present in a subterranean formation, or they can be introduced into a subterranean formation in the course of drilling and completing a wellbore. The elevated temperatures of the subterranean environment can readily promote rapid bacterial growth. In addition, a number of treatment fluids contain materials that are ready food sources for some bacteria, which can further exacerbate bacterial growth issues.
Due to a number of undesirable consequences, it can often be desirable to suppress the propagation of bacteria within a subterranean environment and/or in a treatment fluid being introduced into a subterranean environment. Growth of bacterial colonies within a subterranean formation can produce sludge or slime within the formation and decrease the formation's porosity. Decreased porosity can lower production of a hydrocarbon resource from the formation. Sulfate-reducing bacteria can be particularly problematic within a subterranean environment, since they release hydrogen sulfide as a metabolic product. Hydrogen sulfide can result in corrosion of downhole metal goods, produce deleterious health effects, and lessen the quality of a produced hydrocarbon resource. When hydrogen sulfide or sulfur-containing organic compounds are present in a produced hydrocarbon resource, for example, costly refining techniques may be required in order to make the hydrocarbon resource suitable for its intended end use. Bacterial growth can also degrade certain components within a treatment fluid, such as viscosifying polymers, thereby changing the treatment fluid's properties and possibly making the treatment fluid unsuitable for its originally intended purpose.
Because of the serious consequences bacteria can produce in a subterranean environment, a number of techniques are used to suppress bacterial growth downhole. Continuous or pulsed ultraviolet light sources may be used for this purpose, but bacterial growth may resume if the bacteria are not all killed or inactivated by the light source. Similarly, chemical biocides may be used to suppress bacterial growth in a subterranean formation or in a treatment fluid. Although chemical biocides can be effective against various types of bacteria, it is often difficult to maintain the biocide in a desired location downhole. For example, chemical biocides can exit the subterranean environment during flowback of a treatment fluid, again allowing bacterial growth to resume if the bacteria are not all killed or inactivated.
Bacterial growth within the recesses of a subterranean formation can be particularly difficult to suppress. Specifically, it can be difficult to deliver a chemical biocide or an ultraviolet light treatment into inaccessible subterranean regions, such as the regions within and beyond propped fractures of a subterranean formation. In addition, conveying a chemical biocide through previously propped fractures can upset placement of the proppant, potentially undoing the effects of a fracturing operation and reducing the formation's permeability.
It should be understood that the various aspects are not limited to the arrangements and instrumentality shown in the drawings.